Media captionIan Marlee, Ofgem: 'We have one of the most reliable networks in the world'

Proposed increases in energy bills to pay for more investment in the UK's gas and electricity networks are insufficient, National Grid says.

The energy regulator Ofgem wants energy firms to spend an extra £22bn over eight years to upgrade its networks.

That means bills, once figures from Scotland are included, will steadily rise to leave them £20 higher in 2021.

But National Grid says the proposed funding is about 20% less than it needs to carry out the work properly.

The proposed increases would raise household bills by about £7 in 2013, climbing steadily each year to reach £15 in 2021.

But Ofgem's approval of transmission plans for Scottish Power and SSE in April this year means that bills will actually rise by an average of £15 a year over eight years and reach £20 in 2021.

National Grid, which will carry out the bulk of the work, said Ofgem's plans "differ substantially" from its own business projections.

"While the information currently available is limited, we believe that these initial proposals will not appropriately incentivise the essential investments necessary to provide safe, reliable networks for the UK consumer and avoid delays to the achievement of the UK's environmental targets," the National Grid said in a statement.

Higher bills

Most of the upgrades will be implemented by National Grid because it runs the high voltage and high pressure gas transmission networks.

This needs to be carried out at a time of global financial uncertaintyLord Mogg, Ofgem chairman

About £15bn of the £22bn investment being proposed by the regulator will go towards the improvement of the electricity network in England and Wales, and the gas networks across the whole of the UK.

The rest, approximately £7bn, will be spent on ensuring the low pressure gas networks, which deliver gas to home and businesses, are reliable and safe.

Energy firms had wanted to spend £21bn on revamping the electricity transmission system and to spend a further £9bn on its gas pipelines.